1. Field of the Invention
The present invention relates to a battery capacity detector, and more particularly to a device for accurately detecting a capacity drop in the power supply batteries used for communication equipment.
2. Description of the Related Art
In a portable equipment for mobile communication such as a portable telephone, batteries are generally used for its power supply. Therefore, the normal operation of the equipment cannot be secured if the battery power is deteriorated. For that reason, there have been proposed devices which urge the user to replace or recharge the batteries by issuing an alarm when the battery voltage (or battery power) drops below a specified voltage level.
A cordless telephone system having such an alarm function is disclosed in, for example, Japanese Patent Laid-Open Application No. 354230/1992 (JP-A-H04-354230). This cordless telephone system includes a master set and a slave set which is carried for use. In the slave set, a battery capacity detector detects the voltage of the batteries which supply power to a communication component of the system which includes a radio portion, a modem, a control portion, and the like. When the detected battery voltage drops to below a predetermined value, the control portion displays in a display portion of the system a signal that indicates a drop in the remaining capacity, and notifies the information to the master set via the communication component, Upon receipt of this signal the master set issues an alarm.
Now, a radio data transmitter, such as a teleterminal, has a function of controlling the voltage of transmission signals in response to communication circumstances. For example, the radio data transmitter has a function of monitoring the electric field intensity of a received radio signal. When the electric field intensity lowers, the transmission voltage is increased successively in accordance with the electric field intensity in order to perform all the time a suitable data transmission corresponding to the electric field intensity. However, when the detector for detecting a drop in the battery voltage, as described above, is installed in the radio data transmitter, the load in the circuit supplied with the power is increased with the increase in the transmission power, causing a drop in the battery voltage. Because of this, in a type of device which detects the battery capacity based on the battery voltage, a drop in the battery voltage is detected when the transmission power is increased, and an alarm is issued sometimes in spite of the fact that a sufficient battery capacity remains.
Similarly, if one attempts to equip the abovementioned cordless telephone system with a function which, for example, controls the transmission power in response to the electric field intensity of the received signal, the battery voltage is dropped temporarily due to the increase in the transmission power. Therefore, the system judges the situation as a drop in the battery capacity, which results in a problem of erroneous display or alarm issuance.
Furthermore, when the battery voltage drops temporarily due not only to a change in the transmission power, but also to a variation in the battery load during communication, a similar erroneous detection of the battery capacity may happen.